1. Field of the Invention
This invention relates generally to interface driver circuitry in a data channel and, more specifically, to a single interface driver useful for two different interface signal protocols.
2. Discussion of the Related Art
In the personal computer art, a substantial market exists for devices and circuits manufactured by others for add-on to existing computer systems. For example, a number of manufacturers produce peripheral storage devices of one kind or another, such as streaming tape drives, external disk drives, optical data storage devices and the like. Most modern personal computer systems provide for connection to such peripheral storage devices, but several different electrical and mechanical interface standards have been established in the industry.
This variety of interface standards or protocols presents a practical problem to the personal computer system user as well as the peripheral storage device manufacturer. For example, interface protocols established for use with IBM personal computers differ completely from those used in the Apple Macintosh. Even for computers within the IBM personal computer family, the Quarter-Inch Cartridge Committee (QIC) has established several bus protocols. For instance, QIC-117 specifies an "AT" signal interface designed for open-collector operation and a "PS2" tri-state interface designed for Complimentary Metal-Oxide-Semiconductor (CMOS) operation. The open-collector signal interface must sink 40 mA instead of the 4.0 mA current sinking required in the tri-state signal interface. Equally substantial differences in signal hysteresis and channel capacitance exist between these two IBM interface protocols.
The accepted solution to this problem is for peripheral storage device manufacturers to create a different device model for each such interface protocol. This approach has several keenly-felt disadvantages, including the economic costs of product model multiplicity, the user inconvenience of selecting the proper device model for a particular computer system, and rapid obsolescence of peripheral devices over time with evolving technology.
Practitioners in the art have proposed solutions to this problem. For instance, in U.S. Pat. No. 4,936,785, Eric M. Krug et al propose an interchangeable adapter module that adapts a peripheral storage device interface card to several different signal interface protocols. Krug et al provide a "uniform" interface through the use of a small interchangeable adapter module or "daughter-board" that is mounted on the primary interface circuit board ("mother-board") to complete the interface circuit. Thus, Krug et al propose keeping an inventory of small daughter-boards to avoid the need for a number of larger and more expensive boards. While this reduces the economic costs of product model multiplicity, it does little to address the prospective user and manufacturer inconvenience and confusion.
Ideally, at least for single-manufacturer computer lines, the outside peripheral storage device manufacturer would offer a single interface circuit suitable for immediate connection to any computer in the line; i.e., for example, a single interface for both the AT and the PS2 device ports. This is a problem of particular importance for streaming tape drives, which are intended for direct substitution at disk drive ports, because the tape drive interface must be indistinguishable from the disk drive interface expected at the computer port. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.